In order to improve the light-resistance of a color image in the color diffusion transfer photographic system, a number of studies on the use of various discoloration-preventing agents have heretofore been made. For example, incorporation of various discoloration-preventing agents into a mordant layer of the color diffusion transfer photographic system is disclosed in Research Disclosure, No. 15162, page 81, left column, line 20 from the bottom to page 82, left column, line 8 (Nov. 1976).
However, when such discoloration-preventing agents are added to a mordant layer (in which an image is formed), for example, by a method as disclosed in U.S. Pat. No. 3,432,300, various problems arise. For example, the density of the color diffusion transfer photographic print obtained decreases. Accordingly, the high quality of the photographic print cannot be maintained.
Furthermore, when materials to be incorporated into an image-receiving layer and a method for use in such incorporation are limited in an attempt to eliminate the foregoing problems, the original object, i.e., improvement in the light-resistance of a photographic print, cannot always be satisfactorily attained.
The foregoing reference, Research Disclosure, No. 15162 also discloses in page 82, left column, lines 9 to 6 from the bottom that light-resistance can be improved by adding tert-butylhydroxy anisole as a discoloration-preventing agent to a white reflection layer of the color diffusion transfer photographic system. In accordance with this method, however, the improvement in the light-resistance is not sufficient. Furthermore, when the photographic print is exposed to light for long periods of time, it turns yellow-brown, significantly staining the white ground thereof.
Japanese Patent Application (OPI) No. 159225/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses that for the purpose of improving the light-resistance, phenol-based antioxidants, such as bisphenols, are incorporated into a white reflection layer. However, even when using this method it is desirable to greatly improve the light-resistance, even though the method greatly improves defects of the prior art method.
Conventional white pigments for use in a white reflection layer are undesirable because they deteriorate the light-resistance of a diffusion transfer color image. It is believed that this occurs because white pigments, such as titanium oxide, themselves have optical activity. Accordingly, they exert adverse influences on the light-resistance of the transferred color image. In order to remove the problem caused by the optical activity, almost all of titanium oxides on the market are coated with at least one substance of aluminum oxide, silicon oxide and zinc oxide depending on the particular application. However, even though surface-coated titanium oxide is used as a material for a white reflection layer for the color diffusion transfer process, sufficiently satisfactory light-resistance has not been obtained. It is, therefore, important to incorporate an effective discoloration-preventing agent into the white reflection layer.